Automatic transmissions experience reduced performance when transmission fluid viscosity is too high due to very cold temperatures because the fluid becomes too viscous or thick to flow readily through the filter media. This results in a reduction in the volume of fluid circulating in the transmission due to increased filter restriction. A reduction in the volume of fluid can starve the pump of transmission fluid so that the shift control valves within the transmission do not operate properly.
In filtering transmission fluid, it is desirable to make the filter media more efficient for removing small particles; however, more efficient filter media tends to increase restriction of fluid flow therethrough which causes flow problems in cold temperature start-ups. Accordingly, there is a need for an arrangement which will allow more efficient filter media for the removal of small particles while not having the drawback of starving transmission pumps of fluid.
Other arrangements may also have these difficulties. For example, lubricating oil used in internal combustion engines may be filtered by filters associated with the lubricating oil pan as opposed to spin-on type filters. At extremely low temperatures, the lubricating oil may so increase in viscosity that it does not pass readily through the filter media and thus the engine associated therewith may be starved of lubricating oil. In addition, lubricating oil filters tend to become clogged and need periodic replacement at intervals usually quite shorter than the intervals for transmission filters. Consequently, if these filters become clogged, there is a need to by-pass the filters as is done with spin-on type oil filters.
While transmissions, such as automatic transmissions and other transmission which utilize oil, and internal combustion engines which utilize lubricating oil, are of primary interest with respect to circulating fluid which has become viscous, there are other types of machinery in which operating or lubricating oil is filtered and in which either cold temperatures or clogged filters result in inefficient or improper operation of the machine. Accordingly, there is a need for arrangements which generally address this problem.